Director Peter Jackson, speaking with film critic Elvis Mitchell before a screening last week of his new World War I documentary “They Shall Not Grow Old,” recalled being offered the chance to make a documentary about World War I, a conflict his grandfather fought in and one that Jackson had a lifelong interest in.

“At the time of the premiere of the last Hobbit movie in London in 2014, the Imperial War Museum invited me to come in for a meeting and they asked if I would be interested in doing a documentary to commemorate the Armistice of the First World War. So that was like in four years time — that was 2018. So it seemed like a long, long time away,” said Jackson as the audience laughed along with him at his underestimation of the time and difficulty of the project. “Easy.”

Of course, if it seems unlikely that the Oscar-winning director of “The Lord of the Rings” and “The Hobbit” films would do a standard-issue war documentary — something available in abundance on the History Channel, say — you are correct. There’s nothing easy on display here; the film, which will have a special Southern California engagement on Dec. 17 and 27 and limited release on Jan. 11, is unlike other war documentaries, which is just what was asked for.

“The only thing the Imperial War Museum said to me,” said Jackson. “They said I could take any approach that I want to the First World War but the mandate was that they wanted me to use their original footage — of which they have hundreds of hours — and they wanted me to use it in a ‘fresh and original’ way.”

So, no pressure.

What followed were years of experimentation using all the technological firepower Jackson had available to bring the footage to incredible life.

Restored and updated using a range of digital, 3D and filmmaking magic, “They Shall Not Grow Old” transforms this 100-year-old, often damaged, footage into something astonishing. The film has vivid color, a richly nuanced soundtrack and innumberable computer adjustments that render the movement of the soldiers onscreen close to what they would have been like in real life.

The once-silent footage also has a voice now. The sound mix uses recorded interviews of World War I combat veterans culled from the BBC and Imperial War Museum archives and an impressive array of restored dialogue — which has been reconstructed with the help of forensic lip readers — now voiced by actors.

Audiences can finally hear what they were saying 100 years ago, which is often disarmingly mundane.

It’s an incredible reconstruction, one that makes this past conflict seem suddenly, viscerally immediate. The things one might read about — the trenches, the rats, the bodies -— are suddenly in front of your eyes, and it’s sometimes horrible.

Without dwelling too much on it, Jackson doesn’t steer away from scenes of carnage and death. Even the kinds of trench life typically handled with some humor — viewers get glimpses of soldiers sitting on a log suspended over over a pit that acts a latrine — are revealed to be worse than imagined. According to the veterans recollections, there is no toilet paper, no running water, no ability to clean up — and those logs occasionally snapped, sending soldiers tumbling into the muck.

While there were no shortages of killing implements like tanks and machine guns and snipers, even the landscape was deadly. The muddy ground created a sucking near-quicksand in which men sank into and died.

The main appeal of the film are its major technical achievements, but one of the film’s finest elements are the recollections of the soldiers, who typically expressed no rancor toward the men on the other side of the trench (except, as one noted, the machine gunners), recognizing them as equal victims to the barbaric conflict in which they’d been enlisted.

But truly, the film is an unforgettable, unprecedented document of the lives of the Great War soldiers, one that impressed even its director.

“The serious truth is I was imagining in my head what it might be like,” said Jackson. “But when it finally came off the end of the pipeline I was absolutely stunned.”